1. Field of the Invention
The present invention relates generally to ophthalmic lenses and refers more particularly to so-called progressive lenses the power of which is variable from one point to another.
As is known, a progressive lens of this kind comprises a concave face and a convex face, and it is usually the convex face which is progressive, this face consequently having in relation to a reference spherical surface a deficiency of material or an excess of material which is characteristic of its progressiveness; mention is made of a reference spherical surface only for convenience of explanation and the localisation thereof will be understood later on.
2. Description of the Prior Art
Up to the present time progressive ophthalmic lenses have most usually been made of mineral glass, starting with a curved glass disc provided by moulding with one concave face and one convex face. By rough grinding there is obtained from a disc of this kind a rough glass or blank having geometrically well defined surfaces on its concave and convex faces. The surfaces rough ground in this manner are then improved by fine grinding. Finally, by polishing these surfaces are given the final polish which they require.
These successive operations of rough grinding, fine grinding, and polishing are well known, particularly for producing conventional ophthalmic lenses of mineral glass.
Furthermore, for the production of a progressive lens of mineral glass it has already been proposed to start with a curved glass blank having one of its faces polished and the other only rough ground, this blank being laid with its rough ground face on a shaping block of refractory material, which block is provided for this purpose with a shaped support surface, the assembly comprising the blank and the shaping block being then heated to a temperature just sufficient to enable the glass of the blank to deform without flowing and to bear against the said block.
By the process comprising this heat treatment and due to the fact that the support surface of the shaping block is carefully selected for this purpose, the final surface desired for the treated blank is thereby obtained on the polished face of the latter.
It is therefore not necessary to finish this surface by fine grinding or polishing, and only the face which originally was rough ground must then be rough ground again to the final shape and then fine ground and polished. For the application of heat treatment of this kind to a mineral glass blank intended to produce a lens whose progressive face is the convex face, the support surface of the shaping block usually used in the process described above is convex, and it possesses the progressiveness which is to be imparted to the convex face of the lens.
In the case of the production by moulding of a progressive ophthalmic lens of organic glass, i.e. of a transparent organic polymer, moulding is usually effected between two dies of curved glass or moulds having opposite one another two polished moulding faces, of which one is concave and the other convex, these dies being connected peripherally by a seal of flexible material.
For the purpose of producing progressive ophthalmic lenses of organic glass it is therefore expedient for one of these dies to bear on its polished face a replica of the progressive surface to be imparted to the crresponding face of the desired ophthalmic lens of organic glass.
As already stated above, it is preferably the convex face of a lens of this kind that should be progressive and it is therefore the moulding die or mould having a concave face which must accordingly be progressive.
If the process comprising utilising a shaping block of refractory material, placing a glass die on this shaping block, and heating the assembly to a temperature just sufficient to enable the glass of this die to deform without flowing, were then to be applied to the production of a die of the kind described in the preceding paragraph, it would be necessary to utilise a shaping block having a progressve concave surface. A concave progressive support surface of this kind is however technically difficult to obtain.